In general, a spark plug used for ignition in an internal combustion engine, such as an automotive engine, includes a center electrode, an insulator that holds the center electrode, and a tubular metallic shell provided outward of the insulator, wherein a ground electrode welded to a front end portion of the metallic shell and a front end portion of the center electrode face each other to thereby form a spark discharge gap therebetween. In use, such a spark plug is attached to the internal combustion engine so that its front end (the spark discharge gap) is located within a combustion chamber.
The above-mentioned insulator is inserted into the metallic shell from its rear end side toward the front end side thereof until a stepped portion formed on the outer circumference of the insulator and facing toward the front end thereof comes into engagement with a stepped portion formed on the inner circumference of the metallic shell and facing toward the rear end thereof. In this state, the circumferential edge of a rear end portion of the metallic shell is crimped radially inward so as to form a crimp portion, whereby the insulator is unitarily fixed to the metallic shell.
When a gas mixture is combusted within the combustion chamber, the pressure of combustion gas of high temperature and high pressure acts on the spark plug. Therefore, in general, on the front end side of the metallic shell, a packing or the like is disposed in a clearance between the insulator and the metallic shell, and a crimping load sufficient for preventing leakage of the combustion gas is applied thereto, whereby the gastightness of the spark plug is enhanced. Meanwhile, in some cases, on the rear end side of the metallic shell where the crimp portion is formed, seal material powder, such as talc powder, is charged into a clearance between the inner circumferential surface of the metallic shell and the outer circumferential surface of the insulator, and a packing formed of metal is interposed therebetween. In such a case, the circumferential edge of the rear end portion of the metallic shell is crimped so as to push the packing placed on a layer of the seal material powder toward the front end side with respect to the axial direction and toward the radially inner side, to thereby compress the seal material powder and press the packing against the insulator, whereby the metallic shell and the insulator are assembled together.
In recent years, as a result of a design for increasing the output of an internal combustion engine and decreasing fuel consumption thereof, the size and diameter of a spark plug have been reduced more and more, and the diameter of the insulator used therein has also been reduced.
Therefore, in the case where a structure in which an end portion of the crimp portion of the metallic shell comes into contact with a side surface of the insulator is employed in such a spark plug having a reduced diameter, the following problem may occur. For example, if, at the time of attachment of a spark plug to an internal combustion engine, an attachment tool, such as a wrench, hits against the insulator of the spark plug and the insulator receives an external impact, the insulator may be damaged or broken at a position at which the end portion of the crimp portion is in contact with the insulator and serves as a fulcrum.
In order to prevent such breakage or the like of the insulator, a known spark plug is designed so that, at the time of crimping, only a packing present inside the crimp portion comes into contact with the insulator, and the end portion of the crimp portion does not come into contact with the insulator (see, for example, Patent Document 1).    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2006-92955